Automatic measuring of cross talk



Sept. 2,1941.

E. P. FELCH, JR

AUTOMATIC MEASURING 0F CROSS TAI-1K 4 sheets-sheo 1 Filed April 3, 1940MAMMA /A/VE/VTOR in FELCH, JR. BV

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4 Sheets-Sheet- 2 E. P. FELCH, JR

Fiied Aprila', 1940 AUTOMATIC MEASURING OF CROSS TALK sept. 2, 1941.

A TTORNEV.

Sept. 2, 1941. E. P. FELCH,l JR

AUTOMATIC MEASURING 0F CROSS TALK 4 Sheets-Sheef 5 Filed April s, 1940lA.1 -1071's swf-ld doo? sssasaa l IV VEN TOR E. P. PEACH, JR.

' ATTORNEY E. P. FELcH, JR'- /AUTOMATIC'MEASURING olf" closs TALK Sept.2, 1941.

Filed April I:'3, '1940 4 sheets-smet" 4 www Patented Sept. 2, '194iUNITED STATES PAT E1\JT ori-ICE 2,254,601 AUTOMATIC MEAsURmG F CaossTALK Edwin P. Felch, Jr.. Chatham N. J., assignor to Bell TelephoneLaboratories, Incorporated, New

York, N. Y., a corporation of New York Application April 3, 1940, Serif#No. 327,570 1c claims. (ci. 17a-115.3)

l MAR 2 0 1945 substantiallyl expedite and simplify4 a study of thefeasibility of certain open-wire lines for broad of broad band carriersystems on open-wire lines,

and as cross-talk increases with frequency, it is imperative to haveaccurate cross-talk data for such development. om experience with lowerfrequency systems, fundamental coupling` co-.

band carrier current systems and a check up of the accuracy of computedtransposition's. l

It is an object of the invention to provide apparatus for expeditiouslymeasuring cross-talk in carrier currentsystems. y

It is another object of the invention to provide a. method of andapparatus for automatically measuring cross-talk over a range offrequencies in broad band carrier current systems.

In accordance with a preferred embodiment of the invention, alternatingcurrent energy whose eillcients for commonopenwire configurations f havebeen evaluated. Computation of the systematic components of cross-talkoccurring withv an ideal configuration as a result of a change in thephase of cross-talk between transpositions constitutes a basis fordetermining the transcarrier `current systems.

The existence of dissymmetries and the extent thereof between open-wirelines can only "be, determined by making measurements on actual lines inthe eld. To obtain adequate crosstalk data so as to evaluate the randomeffects of small variations from ideal open-wire configurations, itwould be necessaryl to make thousands of measurements from which theeffects of sag differences, sleet, submarine section-s and treewirecould be determined. Not only would such measurements be helpful inascertaining the ac 'curacy of calculated transpositions but they alsowould be of considerable assistance in determining the feasibility ofincorporating certain openwire lines in broad band carrier systems. Arealization of the magnitude of .such field program maybe had when it isconsidered that in many cases 250 cross-talk frequency curves are`required for complete data on each repeater section of 60 to 100miles`in length. Point-by-point measurements are of doubtful value inascertaining the trend of such curves unless measure- 4 ments are madeat a relatively large number of yposition of open-wire conductorsembodied in y frequency variesover a range within which crossitall:measurements are to be made is applied to one end of a disturbing pairin an open-wire carrier current system 4and a4 portion of such energy ispassed as cross-talk to a disturbed pair extending side by side with thedisturbing'pair on the same row of poles. The energy received e at theopposite end of the disturbing pair is utilized to control theproduction of other alternating current energy varying over a differentfrequency range and having a constant frequency difference therefrom. Aportion of the other energy is heterodyned with the cross-talk receivedAat the opposite end of the disturbed pair to produce a certainheterodyned component of constant frequency throughout the frequencyrange of cross-talk. This component is preferably demodulated to effectan audible alternating current component having a constant frequency andwhose amplitude variations represent cross-talk at .each frequency overvthe frequency range of the cross-talk to be measured. Suitableapparatus responsive to suclpamplitude variation's'is employed forrecording or indicatcross-talk measuring apparatus that would both ingpurposes. I

A feature of thevinvention is that both recording and indicatingA areaccomplished automatically over the range of alternating current wavesapplied to rthe one end of the disturbing pair. Another feature is thatwhile cross-talkhas a different frequency at each successive instant theother alternating current waves to be heterodyned therewithl also have adifferent frequency at each successive instant but the frequency difisensitivity'so as to be most eiiective within a narrow range offrequencies.

The preferred embodiment ofthe invention hereinl disclosed willbe'understofod by'reference to the following description taken togetherwith theA accompanying' drawings, in which:

Fig. 1 illustrates schematically -twopairs of in parallel, either-one orboth of which maybe used as preferred. Let it be assumed that initiallyfar-endcrosstalk measurements are to -be made. By definiconductorsbetween which cross-talk is to be'j measured automatically -inaccordance with the invention;-

Fig. 2 is identical with Fig. 1 except that it shows the invention ingreater detail;

Fig. 3 represents a sensitive arrangement for automatically controllingthe output of an alternating current wave generator'thereby tuning the'receiving terminal over a-rangeof measur-x ing frequencies;

Figs. 4, 5, 6 and v'1 illustrate the operation of Figs. 1, 2 and 3;

v Fig. 8 shows an alternate form of Fig.. 3;' and' Fig. 9 represents theaction in Fig. 8

I Fig. 1 is a'condensedschematic representation of the apparatusemployed for automatically measuring cross-talk passing from adisturbing pair to a disturbed pair both of which conductor pairs extend'side by'side in an open-wire line such level will not be onthe same rowof poles between al sending terminal at'the .left and a receivingterminal at the right.V The distance between these terminals may beequivalent 4to a. repeater section of 60 to 100 miles in length,`whichsections when joined together through their respective repeatersconstitute a complete open-wire transmission line. Each conductor' pairmay accommodate a plurality of carrienchannels depending on thefrequency band utilizedl foreach channel. and the separationtherebetween.

Atthe sending .terminal an oscillator I5 is applied to the disturbingpair while the disturbed pair-is terminated in a suitablenetwork I6. The

put over a desired range which in the present `illustration is from10.to 150 kilocycles within :D5-decibel variation. Also, it is equippedwith a motor driveso as to be driven synchronously through its range ata rteof 5/3, 1 or 3 kilocycles per second. Although not shown, thesynchronous drive embodies facilities for limiting the frequency rangeto the desired band, a warble condenserfor varyingthe output frequencyover a 3-ki1ocycle band about the indicated 'mean value at a rate of 6complete cycles per second, and an intermittently actuated contact lforproviding frequency. reference marks on record paper embodied inreceiving apparatus in a manner that will be subsequently explained.

The receiving terminal of the disturbing pair is connected through atransformer I1 and a variable equalizer I8-'to an automatic frequencycontrol apparatus I9 whose function will be presently described.Bridging the secondary winding 'of the transformer I1 is oneside of a60- decibel calibrating loss network 20 whose opposite sideis connectedto one pair of end terminals 2l, 2| of a double-pole double-throw switchv22, whose other pairlof end terminals 22, 2 3 is' y sending oscillatorI5 is preferably a heterodyne type and is arranged to provide a constantout-1 tion, far-end cross-talk is the ratio of signaling energyappearing at the receiving 'terminal of the disturbed pair to thatappearing at the rewith frequency.

As. the detector 21 measures only a lute levels rather than ratios, itis first necessa to modify the flat gain-frequency characteristic'thereof to complement the loss-frequency characteristic of thedisturbing circuit. vThis is accomplished by means of the adjustableequalizer 26 whose function is well understood and is describedgenerally inthe patent of Zobel, No.

1,603,305, issued October 19, 1926. This means that the level of themeasuring waves supplied to-the detector 21 at the receiving terminalvia the disturbing pair, loss network 20 in its itero4 position andswitch z2 in its right-hand position would be the same as if theequalizer 2l ,Wie omitted and the oscillator ,I5 were controlled to lcompensate for the loss-frequency characteristic of the disturbing pair.In either case, the result is the same, 'that is, measuring waves ofl aconstant level would be supplied to thedetector 21. A similar purpose isfulfilled by' variable equalyizer I8' disposed in the input of theautomatic frequency control I9.

v The recorder 28 and indicator 29 are calibrated i vin cross-talkunits, and such readings are obtained by actuation of the'calibratingloss network 2l in a manner that will now be explained. By definition,one cross-talk unitA equals a 120- decibel power ratio Ibetween adjacentdisturbing and disturbed pairs embodied in an intelligence transmissionsystem. In other words, this means that for such circuits, having equalimpedance,

a 1/1,000,000 part of the current in a disturbing circuit is transferredto a disturbed circuit. Therefore, the loss network 20 is initiallyactuated so that the entire 60-decibel loss is inserted in the circuit,assuming the switch4 22 is closed in the right-hand position. 'I he gainofthe detecto'r 21 isv then adjusted until a reading of 1,000 cross-talkumts is p'roduced on the visual indicator 28. Thereafter, the lswitch 22is actuated to theleft-hand position thereby removing the loss network2l fromthe circuit and applying the disturbed pair through the variableequalizer 2i to the detector 21. The recorder 2l applied through atransformer 2l to the receiving lterminal of the disturbedpair. andwhose center terminals 25,` 25 are applied through anl vother variableequalizer 26 to the input of a heterodyne detector 21. The latter isalso connected to the automatic frequency control apparatus I9. Theoutput of the vdetector 21 is supplied to a recorder- 28 and a visualindicator 2l and indicator 29 are of cross-talk units.

The measuringffrequencies supplied by oscillator i5 are transmitted overthe disturbing pair and applied through the variable equalizer Il to nowboth calibrated in terms the automatic'frequencyl control Il. 'A portionof such energy passes as cross-talk into the disturbed' pair and isapplied through the variable equalizer 2% to the detector 21 whose'output' is divided between the recorder 28 and indicator 29; f Thus. avisual representation 'of the crosstalk passing fr om the disturbing tothe disturbed pair isprovided by the indicator 2l which may aas-gooi l iright-hand edge of a chart embodied in the re corder 28 in a manner thatwill now be described.

' The arrangement for producing .identifying marks on such chart is wellknown and briey comprises a film driven by the synchronous motorassociated with the oscillator I so that at predetermined intervals aperforation in the film allows the closure of an electrical contact andthereby the completion of a discrete electrical circuit, not shown,which extends between the sending and receiving terminals and embodies asolenoid and plunger both of which are associated a with the recorder28. Completion of such circuit energizes the solenoid which actuates theplunger to mark the chart. Passage of the film over the perforationserves to open the electrical contact to cause a deenergization of thesolenoid which then permits the plunger to return to its normalpositionto'await `the next actuation.. In this illustration, a singleidentifying mark is produced at each kilocycle point and threesuccessive marks at the respective 50, 100 and 150- kilccycle points. Aiilm varrangement that may be modified to accomplish the above isillustrated in the patent of T. 'SlonczewskL No. 2,058,641, issuedOctober 27, 1936.

The ,automatic frequency control I9 actuates the detector 21 such thatas the frequency of cross-talk supplied to the latter varies at eachsuccessive instant, the detector 21 supplies alterhating current waveshaving a predetermined constant frequency to the recorder 28 andindicator 29. In other words, as cross-talk varying from 10 through 150kilocycles is applied to the input of detector 21, the automaticfrequency control I8 is arranged to supply thereto at the same timeother'alternating current waves varying in frequency between 475. and615 kilocycles and having a constant frequency difference from the 10 to150-kilocycle crosstalk over -the range thereof. The detector 21heterodynes the respective 4'15 to 615 and 10 to 150 kilocycle ranges ofthe respective other waves and cross-talk to effect a 1kilocyclealternating current wave which throughout the 10 to 150-kilocycle rangeof cross-talk is'supplied to the recorder 28 and indicator 29. Thus, theautomatic frequency control I9 serves to tune automatically the detector21 over the frequency range of- 10 to 150- kilocycie cross-talk so thata continuous and instantaneous measurement of the latter may beeffectedy in terms of variations in the amplitude of the lfkilocyclewave, either on the .chart in- I28 or visuallyl on the included in therecorder dicator 29. y

Fig. 2 shows in further detailthe organization of the automaticfrequency control I9 and the detector 21. Thus it` is seen that theautomatic frequency control I9 includes an amplifier 35 embodyingdelayed automatic volume control and whoseoutput is supplied to amodulator 36 which `may be of a suitable type. From the outacteristic.

put of the latter a predetermined modulation4 component'may be selectedby a nlter 31 and and a filter 39 to the input of a crystal fre-f quency'discriminator 40 whichcontrols, during certain intervals, the magnitudeof a direct current voltage in response to changes in the frequency ofthe predetermined modulation component. 'I'his direct current voltagecontrols sweep circuit 4I which in turn actuates a reverse feedbackoscillator 42 to produce Athe 475 to 615- kilocycle range of otheralternating current waves in a manner that will be presently explained.These latter waves are amplified in isolating-amplifier 43 whosefunction', in addition to amplification, is to preclude reaction of thecircuits to which its output is applied upon the controlled oscillator42.

One portion ofthe 475 to 6l5kilocycle output of the'amplier 43 issupplied through amplifier 44 to themodulator 36 to be combined thereinwith the 10 to 150-kilocycle measuring waves during an interval when thelatter-are being received over the disturbing circuit whereby thepredetermined modulation component which, in 4 this illustration, is465A kilocycles, is effected. Variations in the frequency of thiscomponent cause changes in the magnitude of the direct current voltagewhich controls the sweep circuit 4I and thereby the controlledoscillator 42 'such that the heterodyning of the portion of the 475 toG15-kilocycle waves and the 10 to 150-kilocycle measuring waves in themodulator 36tends to maintain the predetermined modulation component atthe 46S-kilocycle frequency. During an interval of no input `of 10 to'150-kilocycle measuring waves to the amplifier 35, the frequencydiscriminator 40 does not affect the direct current voltage and henceexerts no influence on `the sweep circuit 4I. Therefore, as it will behereinafter pointed out, the latter merely serves 'i to sweep thecontrolled oscillator 42 through its 475 to'615kilocycle frequency rangein a cyclic manner. During an interval of application of 10 to-kilocycle measuring waves to the amplifier 35, thefrequency-discriminator 40 serves (a) to arrest the sweeping action ofthe sweep circuit ranged with suitable networks, not shown, to

reduce by negative feedback the response of the detector 21 to a465-kilocycle component produced in a manner that will be presentlymentioned;I In addition, amplifier 50 embodies a network, not shown,preferably to reduce the response of the detector 21 to the upperside-band components produced in the modulator 46. Also, it is to beunderstood that amplifier 50 `is provided with a fiatfrequency-attenuation char- The amplified 1,0 to 150-kilocyclecross-talk in the output of amplifier 58 and the other portion of theamplified 475 to 615--ki1ocycle waves in the output ofthe amplifier 45are heterodyned in the' modulator 46. Thus, the automatic. self-tuningof cross-talk hereinbefore referred to is effected by the automaticfrequency control la in response to the 1o to 15o-kilocyclemeasuringwaves tranSmitted on the disturbing current. initially to' a.frequencywhich is lslightly below 475, kllocycles, principally bycircuit capacitance? A resistance Il in bridge of the winding 10 servesto provide for' the inductance 11 a Q of subf onant network 14 willoscillate at thatfrequency conductor pair. Manual operation may be acilcomplished by replacingthe controlled oscillator 42 with a suitablemanually controlled oscilator,

notshowm 1- A crystal filter Il having preferably a 10G-cycle band widthselects a predetermined modulation component which' in thisillustrationfhas a fre' quency of 465 kilocycles. This component is atwhich such voltages occur., The resonant networkl 14l will notosclllateat the frequencies at which there is a. phase shift' betweenthe voltages applied thereto through the two 4parallel paths' mentionedabove. Consequently, oscil-l impressed through an amplifier I2 on ademodulator S to be heterodyned therein with a 466- klocycle alternatingcurrent wave furnisheddby an oscillator, From the output of thedemodulator IS, a tuned' amplifier 65 selects a 1 -kilocycle heterodynedcomponent which, after amplification, is 'utilized either in recorder 2lor in' amplier-detector 51 Aand visual indicator 2l, both of whichoperate essentially along the lines shown in the patent of F. E.Fairchild, No. 1,914,414, issued=June 20, 1933.

The frequency discriminator 40, sweep clrcui 4i and controlledoscillator 42 comprise a coni `trol arrangement to providethe wavesextending from 475 to 615 kilocycles. Referring to Fig. 3,

fthe frequency discrlminator 4| includes input terminals I5 andconnecting a source of1 alternatlngcurrent waves, not shown, through arelatively high resistance S1 tothe input grid and cathode of tube VTi.A second grid is applied to lations of the resonantfn'etwork 14 aredetermined by acondition of zero phase' shift between the two'voltagesapplied thereto by way of the two previously pointed-out lmsrallel.vpaths.

'Ihe voltageapplied to the resonant network 14 vla thehereinbeforermentioned second path may be varied by changing the screengrid-anode transcon'ductance of the control tube VTS. This 1 isaccomplished b'y adjusting the direct current bias impressed 'on thecontrol grid'thereof by varying the'charge on the sweep capacity.12. Aspreviously mentioned, theresonant network 14 will osclllate at thatfrequency at which the two toltages supplied thereto have zero phaseshift therebetween. This`may be seen in `li'lg. 4

in which it will be observed that approximately at 64 volts 'applied tothe controlgrid of the control tube VTS, zero phase shift between thetwo about 500 kilocycles; and also at 85 volts applied biasingresistances Il and IS.- AA pieza-crystal 10 shunts the input grid andcathode. The positive terminal of a B battery supply is impressed'.explained.

controlled oscillation' n induces a Lcontrol tube VTS and a resonantnetwork 14 including in parallel a capacity 15 and winding 16 of aninductance 11. The resonant network 14l is applied to the anode of thecontrol tube VTS and the screen., of the `tube VT4 whichv screen func#-tions as an anode. 'Ihe winding 1l of the inductance 1S is connected tothe control grid of the tube VT4. Disposed in the anode circuit of thetube VT4 is an inductance 1S which resonates with the circuitcapacitance below the' frequency of the oscillation of the circuit andhence exhibits the negative reactance character- 3o may be plotted inFig. 4to show the control grid 'potential of VTS and the correspondingfrelvoltages supplied to the resonant network 14.via

thje aforedescrlbed two parallel paths occurs at to the control grid pfthe control tube VTS, a

similar condition obtains at about 600 kilocycles.'

It'will be understood that additional curves quencies at which'the twovoltages supplied to the resonant network 14 have zero phase shifttherebetween. Therefore, it will be obvious that the .constants of theresonant, network 14 and the potentials applied to the control grldpfthe control tube VTSmay be arranged such that the 'resonant network 14will oscillate over a desired range of frequencies. For the purpose ofthis illustration a -to 90volt range of grid potential for the controltube VTS will effect in the output of the tube VT4 a frequency rangewhich extends kilocycles. This is shown in F18. 5;v l

Assuming no input is applied tu the terminals and Il in Fig. 3, thesweeping -capacity '12 is slowly charged through the anode resistance 1ifrom the-'B battery' supply' associated with th'e anode of tube-VTl,until discharge is instituted in the gas tube VT2. Thereupomthe capacityi12, discharges rapidly through the low impedistic of a capacitancewhile, at the same time,

providing a direct' current path in the anode The resonant 4network l14vis tuned stantially 5. The anode of oscillator tube VT4 tube VT4.

In the controlled oscillator 42 there are two parallel paths, ailrst'path comprising the. wind- .ance-pf 'the discharged gaseous tubeVT2until the extinction voltage thereof is attained, whereupon vthe gastube VT2 will be returned to the non-conducting condition. Thus, thecycle'of charging and discharging the capacity 12 may be ing 1I, controlelectrode and screen grid of .the i oscillator tube VT4, and theresonant network- 14, and asecond path embodying the-l anode of theoscillator tube'VT4, screen of the control tube VTS, the resonantnetwork 14 and the screen -of the tube VT4. Consider now two voltagesimpressed on the resonant network 14 'via the aforesaid two parallelpaths. When there is repeated until interrupted in a manner that will bepresently-explained. During each cycle, the

capacity 12 effects a aero to 30-volt variation in the potentialimpressed on the control grid of the tube VTS.v Referring to Fig. 5, itis 'seen thatsuch voltage variation is adequate to sweep .the controlledoscillator 42 over its frequency range of 4'15 to 615 kilocycles..Tlierepetitio'n of these 'cyclesmay be referred to as the Vhuntingaction of thesweep circuit 4I. The sweep circuit constants are suchthateach sweep cycle is approximately one-second duration which isdetermined by the time required to charge the capacity 12 tothebreakdownvoltage of gasa-- ous tube VT2. j

.During the interval of no input to the termi:-

nals 65 and 6C in Fig. 3, the screen grid. of VTI zero phase shiftbetween such voltages, the resis normally biased by the voltage' acrossthe adeffect on the charging and discharging of ,the

sweep condenser 12 and therefore no effect on the hunting action of thesweep circuit 4|. vA

criterion for determining the adjustment of the resistances 68 and 69 isthat the sweep circuit 4| should function once per second` as statedhereinbefore Too low a bias and hence too low a plate impedance tends toreduce the voltage ofcrystal 10,. such wave is also impressed on theinput of the tube VTI. In the latter this causes a rectification actionin the anode-cathode circuit. Such action -serves to increase the flowof space current and to lower the impedance of the y anode-cathodecircuit. This results in a corresponding decrease in the impedance ofthe anodecathode circuit shunting the sweeping capacity 12, aspreviously described. Now, the charging current suppliedthr'ough theresistance 1| from the B battery source is divided between theanode-cathode circuit of the tube VTI and the sweeping capacity 12.Consequently, the voltage across the sweeping capacity 12 is reducedto-a value which is less than that required to institute discharge inthe gaseous tube VT2.v Hence, the hunting action `of the sweep circuit4| is arrested and therefore the gaseous tube VT2 rests inanundischarged state. Now, the voltage across the sweeping capacity 12applied tothe control-grid of the control tube VT3 is 'entirelydependent on the frequency of the alternating current Waves appliedacross the terminals 65 and 66.

The impedance of the piezocrystal 16 between series and parallelresonance is a critical functionA of frequency. Assuming the voltageimpressed thereon is supplied by a constant voltage source, such as theintermediate frequency amplifier 38, Fig. 2, inseries with the'500,000-ohm'resistance 61, Fig. 3, the voltage impressed across thepiezocrystal 1I! isa critical function of frequency. Accordingly, thevoltage applied across the sweeping capacity 12 is also a criticalfunction of frequency. As the voltage of the sweeping capacity 12 isimpressed on the' control grid of the tube VT3, the frequency of theoutput of the oscillator tube VT4 is therefore a critical function ofthe frequency of the alternating current Wave supplied across theterminals 65 and 66, Fig. 3. Referring to Fig. 6, 0 point corresponds tothe maximum impedance of the anode-cathode circuit qf the tube VTI,occurring at the parallel resonance frequency of the piezocrysta110,that is, at 465 kilocycles. Positive and negative varia? tions in the465-kilocycle frequency are reflected as further impedance variationscorresponding to certain voltage changes in the grid-cathode input ofthe tube VTI.

The frequency discriminator 40 is so sensitive that a deviation of `5cycles or less in the 465- kilocycle modulated component applied acrossthe terminals 65 and 66 and thereby across the piezocrystal 10 issufficient to provide such variation in the impedance of theanode-cathode circuitofVTI that the variations in the charge on'thesweeping capacity 12 changes the bias ,in the control'grid of thecontrol -tubeVT3 an amount that is adequate to sweep the output of theoscillator tube VT4 over its entire range -of 475 to 615 kilocycles. Thefrequency discriminator 40 maintains the output of the oscillator tubeVT4 within 5 cycles of the proper frequency over the entire range ofmeasuring frequencies,

that is, over l0 to 150 kilocycles. The control action is rapid enoughto follow i1.5kilocycle warble six times per second. Fig. 5 shows thevariations in the bias on the control grid of the tube VT3 inresponse tochanges in the testing frequency of l0 to 150 kilocycles to providevariations in the output of the controlled oscillator 42 so that amodulated component substantially having a frequency of 465 kilocycleswill be applied to the input of the frequency discrimi A ator 40, Fig.2.

Filters 8| and 82.'in Fig. 3 comprise a 465- kilocycle rejection filterto prevent any 465- kilocycle modulated component from reaching thecontrol tube VT3 and causing any instability thereof.

A threshold arrangement embodied in the dis-- a criminator 40 precludesthe controlled oscillator 42 from tuning to any signal below-apredetermined minimum level. As `the 465kilocycle component applied tothe discriminator 4I) has substantially a constant level effected by theamplier 38 as hereinbefore mentioned, and as such level is severaldecibels above spurious signals and noise, the threshold arrangementensures against false tuning of the controlled oscillator 42. This is sobecause the voltage produced across the adjustable resistances v68 and69 so biases the screen grid of the tube VTI in Fig. 3 thatrectification in the latter cannot commence until such biasing voltageis overcome by a voltage equivalent at least to the level of a propersignal. As the level of spurious signals and noise is below that of aproper signal, it is obvious that the controlled oscillator 42 willrespond only to the voltage of proper signals.

It is to be understood that the voltage applied to the input of tube VTIneed not be derived exclusively from a piezocrystal and further may be adirect current voltage as Well as an alternating current voltage. Forexample, such voltage y maybe derived from phase or level sensitiveapparatus, or the output of a bridge network or potentiometer circuit.In addition, acoustic, electromagnetic, photoelectric or' radio pick-updevices may also be utilized to supply such voltage. Furthermore, thevoltageY across the sweeping capacity 12 is not necessarily limited tothe control of an oscillator but with the addition of suitableintermediary apparatus may serve to balance-'either a bridge. network ora potenthe movement of a boat, an airplane, a tank or a torpedo, and todirect the firing of a gun.

vVTI is within the restricted control range so that the sensitivecontrol comprising the tube VTI and sweeping capacity 12 may seizecontrol of the controlled oscillator 42 in response toa voltage v'applied to the input of VTI, accomplishing at the same time thearresting of the hunting action of the sweep circuit 4I.

The capacity 1l connected acro one electrode and the anode of thegaseous tube VT2 functions (a) to store up a charge wh'enthe gaseoustube VT2 commences to discharge and .to maintain such discharge for aslightly longer interval of time which means that the voltage applied tothe grid of VTS is held at its lowest value for a slightly longerinterval of time, and (b) being imperfect and having a finiteconductance to take a charge which is higher than that normally requiredto break down the gaseous'tube VT2. Essentially, this hasv the effect.of reducing the breakdown value of the gaseous tube VT2. In

this illustration, the breakdown vvalue of the gaseous tube VT2 is madesubstantially 110 volts sausaeoiv Y this .465-kamen nerodyned componentdue.

for example, to a change in the frequency ofthe 10 to 150kilocyclemeasuring waves is reflected as a change in the impedance of theanode-cathode circuit of the frequency discriminator 4I and therefore asachange of thecharge on the capacity 12, which charge, as previouslyseen,

serves to control the frequency of the waves produced by the controlledoscillator 42 such that the heterodyned component applied to thefrequency discriminator 4l tendsto maintain the and the extinctionvoltage about 60 volts. This provides a Sil-volt dilferential .which ismore than adequate to sweep the controlled oscillator 42 over its 475 to615-kilocycle range of alternating current waves while, at the sametime, allowing suilicient B battery supply to eifect the operationthereof.

Fig'. 'l shows the wave form of the sweep voltage produced by thecharging and discharging of the sweep capacity 12. It is saw-tooth inform, sweepingthe output ofthe controlled oscillator 42 over its 415 to615-kilocy'cle range of frequencies, Fig. 5, ata uniform rateasthecapacity 12 is being gradually charged until the breakdown voltage ofVT2 is attained whereupon the capacity 12 rapidly discharges.. 'I'hedlscharge.action of the sweeping capacity-12 is suiliciently rapid topre-- vent control of the controlled oscillator 42 on the downwardfrequency sweep. The A-second interval represents the aforementionedtime funcvtion ofthe capacity 13.

Accordingly, the operation of Fig. 2 is as fol. lows:

During an interval of no transmissionof the z 1Q to 150kilocycle rangeof measuring waves on the disturbing pair and therefore an interval offrequency of 465 kilocycles. In other words, the frequency diilerencebetween the l0 to 150 and 475 to 615kilocycle waves heterodyned in themodulator 36. at a giveninstant is substantially maintained at 465kilocycles throughout the l0 to-150-kilocycle range ofmeasuringfrequencies..

During the same interval of transmission of the l0 to 150kilocyclemeasuring waves, a portion of the 475 to' 6l5kilocycle waves issimultaneously supplied to the modulator 46 embodied in the detector 21for heterodynlng with the 10'to 150kilocycle cross-talk being receivedthereby on the' disturbed pair. The modmator u produces a heterodynedcomponent having a frequency of 465 kilocycles, which component-isdemodulated with` 466kilocycle' waves to elIect an audible 1kilocyclecomponent whosevariations in amplitude'are utilized for automatically4and instantaneou'sly representing cross-talk lover the 10 to150kilocycle range of measuring waves and4 whose frequency is maintainedsubstantially constant over such range as pointedf o ut above inconnection with Fig. 1. The time interval required to complete suchmeasurement is approximately 'seconds.4 In other words, the .automaticfrequency control IS serves to supplythe 475 to 615-'kilocycle range ofalternating current waves to the heterodyne detector 21 such that at noinput of such waves tov the automatic frequency control IS and obviouslyno input to the frequency discriminator 4I),l the sweep current 4I is4arranged to actuate the controlled oscillator? lmatic frequency controlIl, the heterodyning of the l0 to 150kilocycle measuring waves and aportion of Vthe'4'l5 to 15kilocycle waves in the modulator li produces aheterodyned component having a frequency of.465 kilocycles. which com'-ponent 'actuates the fiequency discriminator 4l initially to arrest thehunting action of the sweep. circuit. and thereafterby means of thelatter 'circit'to control the frequency of. the 415 to l 6l5-kilocycle'waves produced by the 'controlled' oscillator '42'. A ny variation inthe frequency of each instant during the transmission of the 10'tokilocycle range of measuring waves the frequency difference between thelatter and the former waves is 465 kilocycles. Thus, the automaticfrequency control VIl serves to 'tune automatically the heterodynedetector -21 to the crowtalk wavesof varying'frequency such that at eachinstant such cross-talk is represented by a component having a constantfrequency and ai corresponding amplitude. The audible l-kilocyclecomponent is particularly useful where observations are to be made withtelephone receivers. However, it is understood that the amplitudevariations of the 465-kilocycle component effected in theheterodynedetector 21 may also be readily utilized inv the recorder 28and indicator 29 to represent cross-talk bytuning the amplifier A55,Fig. 2, to' the frequency of such component; A

Fig. 8 shows an alternate arrangement .for controlling the action of thesweep circuit 4I which action is identical with that described above inconnection With-Fig. 3 except in ther respect that a tunedcircuit 84 isconnected to the control gap C-B of the gaseous discharge tube'VT2 andto the output of the oscillator tube VT4 by a lead 85. a The breakdownpotential of the gap A-'B of the gaseous tube VT2 is normally volts butwhen apotental of '10 volts .is applied across the c'ontrol gap C-B, themain gap will break down at some potential` above 70 volts. Thesweep'capacity 12 is charged vthroughthe resistance 1I until the outputfrequency of thecontrolled oscillator 42 -reaches point E in'Fig. 9-atwhich time an alternating current potential is built up across" thetuned ing potential provided by a battery 86, is adequate on positivepeaks to break down the 'control gap C`B, and hence the main gap A-B.The` latter in the breakdown condition enables the capacity 12 todischarge therethrough until the pointF in Fig. 9 is attained, whichpoint corresponds to the main gap A-B sustaining potential of 'I0 volts.Repetition of this cycle constitutes the hunting action of Fig. 8 andthe latter action continues until a signal impressed on thediscriminator 40 takes control in the manner set forth above concerningFig. 3. l e

Although the invention is particularly described with reference to anautomatic measurement of far-end cross-talk, it is not necessarilylimited thereto'and may be used with equal facilityto measureautomatically near-end crosstalk, noise, transmission and impedance andre-v turn loss. in the latter case a portable bridge is also required.Further, it is to be understood that the invention may be readily usedin accomplishing the above measurements in` coaxial and square-fourcables.

The illustrated apparatus is normally operated from a regulated powersupply which may be either 50 or 60 cycles when such isaVailable.

- In locations too remote from readily accessible commercial powersupply, a portable gasoline engine-generator is adaptedl to -furnish therequired power. For held-service, apparatus according to the invention,including av suitable gasoline engine-generator, is mounted on a truckor trailer, or the measuring apparatus alone may be packed in speciallydesigned trunks` andv shipped to various geographical points. In view ofthe fact that cross-talk curves over a desired range can be drawn atleast in 40 seconds, as previously pointed out, the same apparatus maybe widely used over an extensive geographical area.

What is claimed is:V t

1. In combination, means to produce alternating current waves, means toso control said wave producing means that alternating current waves of apredetermined frequency range are pro-A duced cyclically, and meansresponsive initially to an input voltage to actuate said controlling 4'aaneen circuit u which potential, m addition, to a aan.

4. In combination, means to produce cyclically a voltage of varyingmagnitude, said means comprising a three-electrode gaseous dischargedevice and a capacity connected across the anode and one controlelectrode thereof so that said capacity is charged to the breakdownvoltage of said device and after breakdown discharged therethrough, andmeans to control the variations in the magnitude of the voltage producedby said producing means. said controlling means comprising a thermionicdevice which has its plate circuit connected in parallel with saidcapacity such that when no signal is applied to 5. In combination, meansto produce alternatl.ing current waves, means to control said wavesecond controlI electrode and circuit means to` means such thatinitially the cyclic production producing means such that-alternatingcurrent waves of predetermined frequency range are cyclically produced,said controlling means comprising a three-electrode gaseous dischargedevice, a capacity connected across the anode and.

one control electrode, a tuned circuit connected to the other controlelectrode, a source of biasing voltage applied through said tunedcircuit to said connect the output of said wave producing means to saidother electrode and tuned circuit, said controlling means arranged suchthat said capacity is charged ata uniform rate to actuate said waveproducing means to produce the predetermined range of alternatingcurrent Waves which waves serve to build up a voltage across said tunedcircuit until at the upper end of the predetermined frequency range suchvoltage causes a discharge across both control electrodes of the gaseousdevice whereupon discharge is instituted across the anode and the onecontrol electrode to discharge said capacity through said gaseousdevice, and means responsive initially to an alternating current inputvoltage to actuate said controlling means such that the charge on saidcapacity is limited to a magnitude less than that required `to institutedischarge across the anode and one control electrode of said gaseoustween two conductor pairs which comprises aping a voltage of varyingmagnitude, said capacity 5 cyclically charging to the breakdown voltageof said device and after breakdown discharging therethrough, and meansresponsive initially to an input voltage to limit the charge on saidcapacity to a magnitude less than that required to discharge said'devicethereby arresting the and thereby variations in the magnitude of thevoltage produced e by the voltage producing means.

plying to one pair alternating current waves whose .frequency variesover a range Within which cross-talk is to be determined, deriving fromthey each instant cross-talk at a single frequency. and

waves-of said rst pair to efi'ect other alternating current waves'ofcontinuously varying frequency v but having a constant frequencydifference therefrom, deriving from both cross-talk in the second pairand said other alternating current waves a constant frequency componenthaving an ampli-` tude corresponding to the cross-talk at each frequencywithin the frequency range'of the crosstalk to bedetermined, anddetermining crossat the far end of said first pair other alternatingwaves whose frequency varies over a range with'- other alternatingycurrent ,waves -a constant frequency wave representing at each instantcross-l talk at a single vfrequency over the frequency range withinwmcntne cross-talk a to ,be deter- Y mined, and means to determine thecross-talk current waves 4whose frequency varies over another range.utilizing both the alternating current' waves received at the far end ofsaid rst pair and a portion of said other `alternating current Awaves tocontrol the generation of said latter waves such that a constantfrequency difference is effected between both said waves over thefrequency ranges of both thereof, deriving from the second pair at thefar end thereof cross-talk resulting from the transmission of saidalternating current waves over said first pair, utilizing saidcross-talk and another portion of said other alternating current wavesto effect automatically a constant frequency component representingateach instant the`cross-talk at each frequency throughout the frequencyrange within which the cross-talk is to be observed, and observing' thecross-talk represented by said constant frequency component.

9. The method of measuring cross-talk between two conductor pairswhich-comprises applying alternating current waves of continuouslyvarying frequency to the near end of aflrst pair,

producing at the far end of said first pair other alternating currentwaves of continuously varying frequency,l deriving at the far end ofsaid rst pair fromboth thealternating current waves received thereat anda portion of said other alterrepresented bysaid constant frequency wave.

12. In combination with two conductor pairs, means to apply alternatingcurrent waves of continuously varying frequency to the near end of afirst pair, means at the far end of said first pair to generate otherIalternating current waves.of continuously varying frequency, meansconnected to the far -end of said first pair and said other w'avegenerating'means and responsive to frequency variations of the wavestransmitted on said first pair and a portion of said other waves t0control said other wave generating means such that a constant frequencydiiference is maintained between both said waves transmitted on saidfirst pair and saidbther waves-over the frequency ranges of boththereof, means connected to the far end of the second pair and saidotherwave generating means to derive from the re,

spective cross-talk and another portion of said other waves a constantfrequency component whose amplitude corresponds to cross-talk at ponent.i

13.-`In combination with two conductor pairs, means'to apply alternatingcurrent waves 'of continuously varying frequency to the near end of afirst pair, means at the far ends of both said f pairs to generate otheralternating current waves nating current waves 'a certain componentwhich has a tendency to vary in frequency, utilizing saidfirst-mentioned component .'to control the production of said otherwaves such that a constant frequency difference is maintained betweenboth said waves receivedand produced at the far' end of said first pairover the frequency 'ranges of both said waves, deriving atY the far endof the second pair from cross-talk therein and another portion of saidother alternating current waves a constantfrequency component having anamplitude corresponding to'said cross-'talk at each frequency of therange within which the cross-- tall: is to be measured, and observingsaid crosstalk represented by said second-mentioned comfrequency andwhose amplitude corresponds to.

said cross-talk at each frequencyof the range within which thecross-talk indications are to be made, and means actuated by saidconstant frequency wave to indicate said cross-talk.

1l. Incombination with two conductor Utili.'

of continuously varying frequency, means to der rive a certain componentfrom a portion of said other waves and cross-tall: at the far end of theto control said other wave generating means in response to the wavesreceived at the far end of said rst pair and another portion of saidother I waves such that a constant frequency difference is maintainedbetween said waves received at the far end of said first pair and saidother wavesthroughout the frequency ranges of both thereof and therebyto provide said' certain component with a constant frequency torepresent at each instant thecross-tall: at a single frequency over ythe frequency range of the cross-talkv to be -determined, saidcontrolling means comprising means for effectively applying acontinuously varying actuating voltage to said other wave generating'.means in response to the continuously varying frequencies of both saidother wavesand said waves received at the far end lof said first pair.-

14. Thecombination according to claim 13 in which said controlling meansembodies a threshold device that precludes-spurious voltages from 'endof saidrst pair.

aifectingthe voltage applied to said other wave generating means, saidthreshold device comprising means for producing a biasing voltage whosemagnitude is at least of the order of mnitude ,of the voltage of saidwaves applied to the near and thereby varyingly said other wavegeneratingv means. v I

16; The combination according to claim 13 in which said controllingmeans comprises means to. sweep said other wave generating meanscyclically over its range of frequencies when no alternating currentwaves, are being applied to thenear end of said rst' pair, means toderive from both said waves received at the far end of said iirst pairand said other waves a certain component having a', tendency to changein frequency in response to the continuously varying frequen- 'cies ofboth said waves, and means responsive initially to said last-mentionedcertain component to actuate `said sweeping means to arrest 'the cyclicaction of said other wave generating means and responsive thereafter tothe frequency variations of said last-mentioned certain component toapply a varying voltage to-said sweeping means and thereby a. varyingvoltage to said other wave generating means.

EDWIN P. FELCH. Jn.v

